I always start the water for my pasta and then work on the cheese sauce while the water is boiling. Be sure to salt the water. Cook pasta per directions on package.
Melt butter in large saucepan. I always add salt and pepper to the butter, just a generous amount of each.
Once butter is melted, whisk in flour - will be very thick.
Add milk to butter/flour mixture slowly - about a cup at the time and whisk constantly so that it mixes well.
Add cheese slowly and whisk in to milk mixture good. Continue adding cheese until all has been added and is mixed wll.
Add philly cooking creme and mix thoroughly.
Add 3/4 cup of milk and 8 oz sour cream, whisking in well. Simmer on low heat until your pasta is finished cooking.
By this time, your pasta should be done. Drain and return to pot - DO NOT RINSE. Putting the pasta back in warm allows it to soak up the cheese sauce.
Pour about 2/3 of your cheese sauce into the pasta, stirring to mix well. IT WILL BE VERY CHEESY. It will also seem like you have too much cheese sauce. As the pasta cools, it will soak up a lot of the cheese sauce.
After about 5 minutes, add remaining cheese sauce and the remaining 1 cup of cheese. Stir well!